1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying apparatus or a printer, and more particularly to an image forming apparatus provided with an image holding member called an intermediate transfer member.
2. Description of the Related Art
In the electrophotographic color image forming apparatus, there is already known a configuration having an intermediate transfer member in addition to the photosensitive drum. In such a configuration, a primary transfer operation of transferring the toner image, formed on the photosensitive drum, onto the intermediate transfer member is repeated plural times to superpose the toner images of plural colors on such intermediate transfer member. The toner images of plural colors then are collectively transferred onto a transfer material such as paper ("secondary transfer" operation).
FIG. 8 shows an example of the conventional color image forming apparatus utilizing the intermediate transfer member.
The color image forming apparatus is provided with a photosensitive drum 101 which is rotatably supported and is rendered rotatable in a direction R1. Around the photosensitive drum 101, there are provided developing units of different colors, namely four developing units 105, 106, 107, 108 respectively containing developers (toners) of black (K), magenta (M), cyan (C) and yellow (Y). These developing units 105 to 108 are in turn brought into contact with the photosensitive drum 101 by unrepresented contacting means, for developing the electrostatic latent images formed on the photosensitive drum 101.
The photosensitive drum 101 is surfacially uniformly charged with a charger 102, and an electrostatic latent image is formed by scanning with a scanning beam (laser beam) 104 from a laser exposure optical system 103. Portions exposed by the laser beam within the electrostatic latent image are selectively developed with the above-mentioned developing unit 105 to 108 and visualized as a toner image. The toner images formed on the photosensitive drum 101 are transferred, by a primary transfer roller 110, onto an intermediate transfer belt 109 constituting the intermediate transfer member ("primary transfer" operation).
The above-described steps of latent image formation, image development and primary transfer are repeated for each of the four colors of yellow, cyan, magenta and black, whereby a color image consisting of four superposed toner images is formed on the intermediate transfer belt 109. Then such toner images of four colors are transferred in the block to a transfer material P, which is pinched and conveyed by a secondary transfer roller 111 and the intermediate transfer belt 109 ("secondary transfer" operation).
In the following there will be further explained the primary transfer and the secondary transfer mentioned above.
Where the photosensitive drum 101 is composed of OPC (organic photo-semiconductor), for example, having negative charging characteristics, the developing units 105 to 108 utilize negatively charged toners to develop the areas exposed to the laser beam 104. Consequently, at the developing operation, the primary transfer roller 110 is given a positive transfer bias by a bias power source 120 constituting the primary transfer means together with the primary transfer roller 110.
The intermediate transfer belt 109 is composed, for example, of an endless resinous film of PVDF (polyvinylidene fluoride), nylon, PET (polyethylene terephthalate) or polycarbonate of a thickness of 100 to 200 .mu.m and a volume resistivity adjusted if necessary to 10.sup.11 to 10.sup.16 .OMEGA.cm, and is supported by a rear roller 112, a driving roller 115 and a tension roller 116.
The intermediate transfer belt 109 consisting of such thin film forms a large electrostatic capacitance of several hundred to several thousand pF at the primary transfer nip portion N1, thereby realizing a stable transfer current.
In another configuration, the intermediate transfer belt 109 is composed, as shown in FIG. 9A, of a base layer 109a and a surfacial layer 109b, in which at least the base layer 109a is composed of an elastic member (hardness of 60.degree. to 90.degree. according to JIS-A measurement) of a thickness of 0.5 to 2 mm.
Such configuration is effective in avoiding hollow images (hollow characters) which are encountered in the aforementioned resinous belt. The intermediate transfer belt 109 with a high surface hardness tends to form a hollow images in the toner image on the intermediate transfer belt 109, but the elastic member 109a reduces the surface hardness of the intermediate transfer belt 109, thereby preventing the formation of hollow images in the toner image.
On the other hand, the toner image supported on the intermediate transfer belt 109 is conveyed over plural turns in the surfacially supported state, and, if the electrostatic attractive force of the toner to the intermediate transfer belt 109 is weak, the yellow, magenta, cyan and black toner images superposed in succession on the surface of the intermediate transfer belt may be distorted when the intermediate transfer belt is bent or the surface thereof repeats extension and contraction over the rollers 112, 115, 116 supporting the intermediate transfer belt.
In particular, if the intermediate transfer belt 109 includes the elastic member, when the intermediate transfer belt 109 is subjected to extension and contraction (in a belt portion, the outer belt surface is extended while the inner belt surface contracts) for example over the roller 115, the magenta toner tM deposited on the yellow toner tY on the intermediate transfer belt 109 receives electric repulsion from the yellow toner tY while being subjected to the shock of bending, extension and contraction of the belt 109, thereby being scattered the magenta toner tM, as shown in FIG. 9B.
Naturally such scattering phenomenon occurs less if the volume resistivity is low, even in the resinous belt not including an elastic member.
Such toner scattering phenomenon becomes conspicuous when the toner image of each color contains a large amount of toner and the toners of plural colors are superposed to form a full-color image on the intermediate transfer belt. This is because, when toner images are superposed on the intermediate transfer belt, the toner image present in the upper layer (toner image transferred last) is easily scattered.
In order to avoid such toner scattering, the present applicant has found a method of increasing the volume resistivity of the resin in the aforementioned resinous belt or of the surfacial layer in the belt including the elastic member, as shown in FIG. 9A thereby forming a potential wall as shown in FIGS. 9C and 9D.
In the following there will be explained the transfer to the transfer material by the secondary transfer roller 111 (secondary transfer).
The secondary transfer of the toner image onto the transfer material P is executed by the secondary transfer means consisting of the secondary transfer roller 111, the rear roller 112 and the bias power source 121. A secondary transfer nip portion N2 is formed by pinching the intermediate transfer belt 109 with the rear roller 112 of a low resistance, positioned inside the belt 109 and grounded or given a suitable bias as a counter electrode and the secondary transfer roller 111 of a low resistance positioned outside the intermediate transfer belt 109. The secondary transfer is executed by applying a positive transfer bias by the bias power source 121 to the secondary transfer roller 111 and causing the secondary transfer roller 111 to abut on the rear surface of the transfer material P.
On the photosensitive drum 101 after the primary transfer, the toner remaining thereafter is removed and recovered by a cleaner 119. The retentive charge then is eliminated by an exposure unit 117 and the drum is used again for the next image formation. Also the surface of the intermediate transfer belt 109 after the secondary transfer is subjected to the removal of toner remaining after the secondary transfer by a cleaner 113, and the removal of the retentive charge by a charge eliminating charger 114, which can be composed for example of an AC corona charger or a roller charger of contact type.
At the primary transfer operation in the above-described image forming apparatus, there may result a defective transfer or a defective image depending on the magnitude of the transfer potential contrast between the positive bias applied to the primary transfer roller 110 and the negative potential of the photosensitive drum 101. With respect to the defective transfer, there exists an optimum transfer contrast according to the magnitude of the negative charge of the toner. The transfer efficiency is lowered when the transfer potential contrast at the primary transfer unit is too larger or too small. However, when resistance of the intermediate transfer belt 109 is elevated in order to suppress the aforementioned scattering of the toner, it is necessary to increase the transfer bias in order to maintain the transfer efficiency, and the increased transfer potential contrast causes discharge in the air in the vicinity of the transfer nip portion, thereby distorting the image.
When the discharge in air occurs in the toner at the execution of the primary transfer, the image distortion appears as a distortion in the toner image on the intermediate transfer member at the primary transfer. In case the discharge in air occurs when a non-image area of a color is superposed on the toner image previously transferred onto the intermediate transfer member, a charge pattern is formed thereon according to the pattern of discharge, and induces distortion of the toner at the secondary transfer onto the transfer sheet after the completion of the primary transfers.
The discharge in air occurs more easily in the latter case of transfer of the non-image area of the next or subsequent color onto the color already formed on the intermediate transfer member, than in the former case of distortion in the primary transferred color itself. This is because the image formation is executed by reversal development, thereby increasing the potential in the non-image area of the photosensitive member in the negative direction (namely dark potential) and accordingly increasing the transfer potential contrast.
Besides, as the resistance of the intermediate transfer belt 109 increases, the optimum point of the transfer efficiency tends to go beyond the range without the above-mentioned discharge in air, so that it is extremely difficult to find a point of simultaneously satisfying the toner scattering, the transfer efficiency and the image distortion (particularly the latter one appearing at the secondary transfer).